There is much to cheer about President Trump's first decisions as president. Conservatives are almost certainly going to love his Supreme Court pick, his stepped-up efforts to deport criminal aliens, the approval of two oil pipelines stubbornly blocked by the previous administration and more.

Trump remains a potential time bomb, however, both because of his genuine, anti-trade beliefs and because of his unwholesome character. Leave aside the distractions about crowd sizes and voter fraud. There is an aspect to Trump's egotism that is more than unseemly or clownish; it's aggressive. He enjoys disparaging and demeaning others personally, often about things (like their appearance or their "time of the month") that few over the age of 11 would stoop to.

These two strands — Trump's misguided view that trade is bad for America, and his penchant for humiliating others — came together this week. The president of Mexico, Enrique Pena Nieto, canceled a meeting with President Trump, signaling an escalation of tensions with our southern neighbor. This is entirely our fault.

It's one thing to stress getting control of our borders. Even those who believe that immigration is a net positive for the nation agree that illegal immigration must be better policed. But cracking down on illegal immigration should mean getting our own house in order.

It should mean policing all of our borders, not just the one with Mexico, and it should mean due diligence about visa overstays. Visitors who overstay their visas amount to at least half, and probably closer to 60 percent, of those entering the country illegally now. They arrive at airports, not across the Rio Grande. The great wave of illegal crossings from the south crested in 2007 and has declined steadily since. As immigration hawk Mark Krikorian noted in 2015, "Border crossings really are way down." Well, some border crossings are way down, others not. More Mexicans cross the border heading south now than north. In other words, net migration from Mexico is negative.

One of the blessings the U.S. has always enjoyed is having good neighbors. As Aaron David Miller put it, "The United States is the only great power in the history of the world that has had the luxury of having nonpredatory neighbors to its north and south, and fish to its east and west."

But that can change. Not content with proposing a totally unnecessary wall across our entire southern border (double fencing is much cheaper, easier to maintain and effective), President Trump felt the need to insult and humiliate Mexico. It began with the accusation that the Mexican government was "sending" the illegals who crossed the border and somehow complicit in their traffic of crime and drugs. In fact, the Mexican government cooperates with the U.S. in fighting drug trafficking and other crimes. Just last week, Mexico extradited drug lord "El Chapo" to the U.S. for prosecution. Further demeaning our friends to the south, President Trump made forcing Mexico to pay for a wall part of his stump speech and has refused to drop the demand.

Naturally, Mexicans are offended and outraged by this insult. "We're not paying for your f——— wall," fumed former Mexican president Vicente Fox. And they're worried, and should be, that Trump has proposed to "renegotiate" the NAFTA trade agreement, which he describes as "the worst trade deal maybe ever signed anywhere, but certainly ever signed in this country."

One reason that fewer Mexicans are attempting to enter the U.S. illegally since 2007 may well be that NAFTA has succeeded in improving the jobs picture there. (Another reason is surely that the birthrate has declined, which always reduces emigration.) Fred Smith, founder and chairman of FedEx, estimates that NAFTA makes the U.S. $127 billion richer every year than it would be without it.

So the two areas of maximum importance to stability and prosperity in our hemisphere, trade and mutual respect, are both under assault by our president.

It is not written in stone that Mexico will remain a stable nation and friendly neighbor. Mexico is under stress due to falling oil prices, corruption and other woes. The peso has lost 10 percent of its value due to fears about NAFTA, and as Jose Cardenas notes in National Review, a left-wing, Hugo Chavez-like demagogue named Andre Manuel Lopez Obrador could stand to benefit from the current uncertainty and understandable resentment.

Good fences may make good neighbors. But this wall, and particularly the way Trump has rubbed Mexicans' noses in it, may turn a good neighbor bad fast.